1. Field of the Invention
The present invention relates to an image processing technology for processing an image picked up by an image pickup section.
2. Description of the Related Art
In recent years, a CMOS image sensor or a like device having a linear characteristic and a logarithmic characteristic is incorporated in an image pickup apparatus such as a digital camera to increase the dynamic range of image data. The following technologies have also been known in relation to the above.
For instance, the article “Contrast Improvement for a Linear/Log CMOS Image Sensor” (D1) in Konica Minolta Technology Report Vol. 4 (2007) discloses a technique for compressing the dynamic range of image data, wherein an illumination component i.e. a low frequency component of image data is extracted based on a space frequency of an image, and the extracted illumination component is compressed, with a reflectance component being left uncompressed.
U.S. Patent Application Publication No. 2006/0188172A1 (D2) corresponding to Japanese Unexamined Patent Publication No. 2005-45514 discloses a technique for correcting a distortion of a picked up image resulting from distortion aberration of a lens by performing linear interpolation with use of vicinity pixels and changing the resolution of the image depending on the image pickup position.
U.S. Patent Application Publication No. 2006/0001061A1 (D3) corresponding to Japanese Unexamined Patent Publication No. 2006-50544 discloses an image sensor operable to linearly convert incident light into an electrical signal until the incident light amount reaches a predetermined light amount, and logarithmically convert the incident light into an electrical signal if the incident light amount exceeds the predetermined light amount.
In the case where an image pickup apparatus is incorporated with one of the arrangements disclosed in D1 through D3, there is a likelihood that a degraded image may be outputted depending on the order of processing. Specifically, observing an image to be obtained before the correction processing disclosed in D2 is performed, the space frequency of an image signal obtained from a central part of a picked-up subject image may be different from the space frequency of an image signal obtained from a peripheral part of the picked-up subject image. Under the above condition, if the dynamic range compression disclosed in D1 is applied to the arrangement disclosed in D2 without any processing, image data corresponding to some part of the subject image may be compressed, and image data corresponding to the other part of the subject image may be left uncompressed depending on the image pickup position.
The image sensor disclosed in D3 has two kinds of photoelectric conversion characteristics, and image data of an image picked up by the image sensor has the two kinds of photoelectric conversion characteristics. Accordingly, if the correction processing disclosed in D2 is applied to the image sensor disclosed in D3 without any processing, pixels having the different photoelectric conversion characteristics may be subjected to linear interpolation, with the result that output image data may have the different photoelectric conversion characteristics.